I, like others, assumed that he already had a JAA PPL.
On that basis, I repeat and am correct, that getting an FAA Private Cert does not require any exams or flying.
None, nada, zip.
It is, equally, simply a paperwork exercise apart from (as I mentioned) having to go to the US to collect the thing... Still no requirement for any exams or flying, also no requirement for any US medical or biennials - indeed, nothing additional to what you already have/need for your JAA (or most other states') licence.
If you then need to use your shiny new licence, there may well be additional requirements (depending on where/how/what) you want to fly - but that was not the question asked.
So, whilst the standalone option is undoubtably better, the piggyback is considerably cheaper and more easily obtained.
Fly safe, whatever licence you use! Sam.